A pluripotent stem cell, which is capable of differentiating into all cell lines of organism, has a potential ability to differentiate into an arbitrary cell type, and generate an arbitrary type of tissue or organ. Accordingly, application of the cell to new therapies of regenerating and supplementing a lost cell in an organism (regeneration medicine) is being expected. Furthermore, the cell has availabilities as a research tool in the field of embryology, molecular biology and pharmacy.
As a pluripotent stem cell, an embryonic stem cell (ES cell), which is a cell strain established from an inner cell mass among an early embryo at a stage called blastocyst, is known. However, a human ES cell prepared by destructing a human embryo is objected frequently from an ethical point of view, and its study is restricted in many cases.
In order to overcome problems of the ES cells, a pluripotent stem cell, which is a cell strain induced by artificially introducing genes into a somatic cell without using an embryo (induced pluripotent stem cell (iPS cell): e.g., Non-Patent Document 1), was developed. The cell is prepared by artificially introducing genes involved in acquisition and maintenance of totipotency of a cell into a somatic cell. Currently, study is actively progressed for procedures for preparing the cell or methods for utilizing the cell.
The Non-Patent Document 1 discloses a method of preparing a human iPS cell by introducing genes encoding four kinds of peptides of OCT3/4, SOX2, c-MYC and KLF4. In addition, other researchers reported that a human iPS cell was prepared by introduction of genes encoding another four kinds of polypeptides of OCT4, SOX2, NANOG and LIN28 (Non-Patent Document 2). However, not all the somatic cells into which these genes have been introduced are induced into iPS cells and, actually, only parts thereof (0.02% or less of the total cell number) is merely introduced into iPS cells. In order to improve this efficiency, addition of an agent inhibiting methylation of DNA to a medium has been attempted (Non-Patent Document 3). In addition, since c-MYC is an oncogene, and a risk of canceration of the gene-introduced cell is unavoidable when the cells are applied to medicine, attempt of inducing iPS cells without c-MYC has been carried out. In this case, however, an induction efficiency is further reduced (0.001% or less of the total cell number). Like this, it cannot be said yet that the technique of preparing iPS cells has been established under current circumstances.